Generally, a conventional CATV receiver system applied within an analog broadcasting systems employs a double superheterodyne system to decrease an image frequency interference signal which is caused by a cross modulation. FIG. 4 shows in a block diagram a double superheterodyne system tuner circuit of the CATV receiver in the conventional analog broadcasting system. In FIG. 4, a numeral 41 denotes a tuner input section of the CATV receiver. A high-frequency reception signal of a desired channel is received through the input section 41 and selected by an up-converter 42. The selected signal is then converted to a higher frequency corresponding to a first intermediate frequency (IF) signal. The first IF signal is down-converted to a second IF signal having a fixed frequency through a mixing process performed by a mixer 43 using a local oscillation frequency provided by a local oscillator 44. The second IF signal is then led to an output section 45. The second IF signal is served as an IF signal of TV signals. An audio signal and a video signal for TV reception are extracted from the second IF signal through a demodulation process.
FIG. 5 is a block diagram showing a tuner circuit of the CATV receiver in a digital broadcasting system. Here, the digital broadcasting system will be exemplified by taking a multi-value QAM modulation system. In FIG. 5 a numeral 51 denotes a tuner input section of the CATV receiver. A high-frequency reception signal of a desired channel is received through the input section 51 and selected by an up-converter 52. The selected signal is then converted to a higher frequency first IF signal therein. The higher frequency first IF signal is split to two signals which are respectively applied as inputs to mixers 53 and 54. Additionally, one of two split oscillation signals that are output from a local oscillator 55 is phase-shifted by 90.degree. through a 90.degree. phase shifter 56 for a QAM detection and applied as an input to mixer 53. The other of the two split oscillation signals output from local oscillator 55 is applied as an output to the other input of the mixer 54 without being phase-shifted. The outputs of the mixers 53 and 54 will be obtained as an I signal and a Q signal, respectively. The I signal and the Q signal are detection signals of the multi-value QAM signal carried by the reception signal. The I signal and the Q signal are led to output sections 57 and 58, respectively. The I and Q signals are used for extracting the audio signal and the video signal in the TV reception through a multi-value PSK modulator and a digital modulator.
The above-described arrangement constitutes the conventional CATV receiver which is compatible to both the analog broadcasting and the digital broadcasting systems. However, in CATV systems, all broadcasting channels are not always transmitted by a fixed one of the digital broadcasting system and the analog broadcasting system. Rather, the analog broadcasting channels and the digital broadcasting channels are transmitted together in the CATV system. Therefore, convention CATV systems require a receiver which is compatible with both the analog broadcasting and the digital broadcasting. Such a receiver, which is compatible to both the analog and digital broadcastings, is realized by the present invention.
Returning again to FIG. 5, when a signal received through the input section 51 is the analog broadcasting signal, a desired channel of that high-frequency analog signal is processed by the up-converter 52 and the selected channel is converted to a higher frequency first IF signal. The higher frequency first IF signal is then split into two signals. When performing analog broadcasting reception, each of the two split signals may only be used for processing the TV signal. For example, here it is assumed that only the first IF signal applied to the mixer 54 will be utilized. For that reason, in case of analog broadcasting reception, the other mixer 53 and the 90.degree. phase shifter 56 are not needed. Thus, during analog broadcasting reception, the first IF signal and the oscillation signal directly output from the local oscillator 55 are mixed in the mixer 54 and then converted to a second IF signal. The second IF signal is served as the IF signal of the TV signal which is to be used for extracting the audio signal and the video signal in the TV reception by demodulating the signal.
The above-described arrangement thus constitutes the CATV receiver compatible to both the analog broadcasting and the digital broadcasting systems by utilizing the digital broadcasting reception system as it is. However, in a CATV receiver arranged as mentioned above, image frequency interference signals have caused a problem, especially in the analog broadcasting reception.
The image frequency interference signal will be explained in reference to FIG. 6. In CATV receivers for the analog broadcasting, a desired reception signal fR and its image frequency interference signal fI, shown in FIG. 6, are received through the input terminal 51. The desired reception signal fR of the desired channel and the image frequency interference signal fI are selected by the up-converter 52. The channels are then converted to higher frequency first IF signals in the up-converter 52. The first IF signals having a fixed frequency are converted to second IF signals in the mixer 54 through a mixing operation with the local oscillation signal from the local oscillator 55. At this time, the image frequency interference signal fI applied to the mixer 54 results in an image IF interference signal fIM which corresponds to a differential frequency between the frequencies of the image frequency interference signal fI and the local oscillation signal fL on the output terminal 58. The image IF interference signal fIM in the output signal on the terminal 58 is then eliminated by a suitable filter for providing only the desired second IF signal fIF to a following stage for processing the second IF signal.
In this way, since the image IF interference signal fIM can not fall into the band of the local oscillation signal fL of the selected channel, such a surface wave TV system having channels spaced apart from each other with a relatively wide interval can eliminate the image IF interference signal fIM using a suitably designed filter which passes the local oscillation signal fL therethrough. However, in the case of the CATV broadcasting, as shown in FIG. 6, there is no space between the channel intervals different from those in the surface wave TV system. Since, in the local oscillation signal IF converted in frequency by the mixer 54, the image IF interference signal fIM falls into the band of the local oscillation signal fL of the selected channel when the reception signal is received, it is difficult to eliminate the image IF interference signal fIM using a filter for passing the local oscillation signal fL. Moreover, it is difficult for the conventional CATV tuner to vigorously depress the image IF interference signal fIM from cost and technical aspects. To provide a countermeasure against the persisting image IF interference signal fIM, a shield for tuners must be intensified. However, since it is impossible to eliminate the whole image IF interference signal fIM, the receiving performance has been deteriorated in the analog broadcasting reception state in the CATV receiver.
As described above, conventional CATV receivers have a drawback that is difficult in cost and technical aspects to fully eliminate image frequency interference signals which are generated in conjunction with a frequency conversion in the mixer.